That brown line along the bottom edge of your solar panels is dried dirt and water. Low pitch roofs don’t let rain run straight off the glass, so a thin puddle sits against the bottom frame, picks up dust and pollen, then dries out and leaves a mark. Left alone, lichen and moss often get their first foothold right on that same line.
It’s a common sight on low pitch roofs, and it’s easy to miss from the ground because it only runs along the very bottom of the panel. Here’s why it forms, what it’s actually doing to your system, and what stops it coming back.
Why does dirt collect in a line along the bottom of the panel?
Every panel has an aluminium frame around the glass, and that frame has a small lip running along the bottom edge. On a steep roof, rain clears that lip easily on its way off the glass. On a low pitch roof, generally under about ten degrees, water doesn’t have enough fall to clear it, so a thin puddle sits there instead.
Rain and dew wash a bit of dust and pollen down the glass (rain does not actually clean panels), and it ends up trapped in that puddle rather than running off the roof. When the water finally evaporates, the dirt it was carrying stays behind as a line. The next bit of rain tops the puddle back up, and the line gets a little darker each time it happens.
It’s worth checking early. That strip stays damp longer than the rest of the panel, which is also why lichen and moss tend to start there first. Our guide to lichen on solar panels covers what that looks like once it’s gone past a simple dirt line, and what it costs to deal with at that point.
Does a thin dirt line actually affect how much power the panel makes?
Yes, and it can matter more than the size of the line suggests. The cells inside a panel are wired together in strings, a bit like a row of fairy lights. The current flowing through a string is limited by whichever cell in that string is darkest. If the bottom row of cells is sitting behind a dried, dirty line, it isn’t just those cells losing output. It drags down the whole string with them.
Panels have small components called bypass diodes built in to stop this turning into a bigger problem. When a group of cells gets shaded or dirty enough, the diode gives the current a way to skip around that group rather than forcing it through. That protects the panel from overheating, but it also means the bypassed section stops contributing anything while it’s happening. A dirty strip covering a small part of the panel can end up costing you more than its own share of the output.
What do drain clips actually do?
Drain clips sit over the bottom edge of the frame and give the water a channel over the lip that was holding the puddle, so it drains away instead of sitting still. No puddle, no evaporation line, and no head start for lichen.
We stock Solarsiphon drain clips through our shop, in sizes to fit most panel frame thicknesses. They clip onto the edge of the frame, so they can go on an existing array as well as a new one, and come off again for cleaning. They suit low pitch roofs where water genuinely has nowhere else to go. A steep roof that already sheds water cleanly won’t see the same benefit.
Does a clean get rid of the line, or do I need clips as well?
A normal clean takes the line off. It’s dried dirt sitting on glass, which is exactly what a soft brush and purified water deal with. Our solar panel cleaning price guide has how pricing works, and a dirt line on its own doesn’t usually change that.
What a clean doesn’t change is the roof pitch. If nothing else changes, the puddle forms again and the line comes back over time. That’s the difference clips make. They don’t clean the panel, they stop the water sitting there in the first place, so there’s less for a line to build from between visits. We’ve seen output improve by up to 30% on some systems we’ve cleaned, and keeping a low pitch array from re-soiling along the bottom edge is part of holding onto a gain like that.
Common questions
What causes a brown or dark line along the bottom of a solar panel?
Water pools against the lip of the frame on low pitch roofs instead of running off. When it evaporates it leaves behind the dirt and pollen it picked up on the way down, and that builds into a visible line over time.
Does the line mean my panels are damaged?
No, on its own it’s just surface dirt. It’s worth dealing with because of what grows there next and the output it costs while it’s there, not because the glass or the cell underneath is harmed.
What panel thicknesses do drain clips fit?
Our Solarsiphon clips fit frame thicknesses of 30, 32, 35 and 40mm, which covers most residential panels. Check your frame depth before ordering if you’re not sure which size you need.
Can you fit drain clips for me?
We sell Solarsiphon clips through our shop. If you’d like them fitted while someone’s already on your roof for a clean, mention it when you book and we’ll let you know whether that’s something we can arrange for your system.
Will lichen definitely grow where the line is?
Not always, but that strip stays damp longest, so it’s usually the first place it shows up if it’s going to. See our lichen guide for what to look for.
How often should I get the line cleaned off?
The same as the rest of the panel. Our guide to how often to clean solar panels in NZ covers what changes that for your roof.
Not sure if your roof is the type that needs clips?
Send us a couple of photos of the panels from the ground, straight on and from the side if you can get it, and we’ll tell you whether the pitch and the line are something drain clips would help with. See our solar panel cleaning service, or get a free quote.